1. Field of the Invention
The invention relates to lids for cans for receiving foodstuff, which are to be subjected to a thermal treatment above 50° C. in the form of sterilization or at least pasteurization. Methods for the production of the lids and for securing the leak tightness of the sealing are also covered.
2. Description of the Related Art
Lids are concerned having an annular ring for seaming to the can body, the lid can be firmly and tightly connected with the can body rim and is in particular made from metal, and a “lid diaphragm” (diaphragm or panel) in the form of a cover surface which is affixed to the annular ring (for seaming to the can body) in such a way that, for the opening of the can, the panel can be pulled off from the seamable annular ring or may be peeled off from it by means of pulling. This relates on the one hand to peeling foils and, on the other hand, also to foils which are to be torn at the edge or are to be further torn.
Various designs of such lid covers are known, see U.S. Pat. No. 4,211,338 (Bublitz). Difficulties with these lids arise at high temperatures, at any rate above 50° C., as they are—as a rule—present in sterilization or pasteurization processes in continuous autoclaves (retorts). The lid will leak or the lid diaphragm even begins to detach itself from the annular ring under the action of a difference in pressure which acts upon the lid, or can be damaged in such a way that this would result in a later leakage at the sealing line.
Consequently, these cans are mostly sterilized in such autoclaves (batch retort stations) which are equipped with means for generating an outer air or vapor pressure in order to apply a sufficient counter-pressure onto an outer surface of the lid, opposing the internal pressure in the closed can, due to which the lid, in particular the lid diaphragm, will be protected against too high differences in pressure AP.
It is not possible or requires great effort to equip “continuous autoclaves” with such counter-pressure means that are suitable for continuous passage.
Continuous autoclaves (for a pasteurization or a sterilization process) with a counterpressure due to a vapour atmosphere certainly generate a low permanent counterpressure on the lid surface of up to 1.6 bar (0.16 MPa), but they are not sufficient for stabilizing customary “peel lids” without damage.
In addition to this, the lid surface does not form any smooth, fine-looking surface after cooling of the cans, which impairs acceptance by the customers and results in illegibility of any type of inscriptions or bar codes by scanners.